Abstract
This study conducts the experimental investigation, visualization and modeling of convective boiling of water in the three types of single microchannel with approximately the same hydraulic diameter of 35μm. The aspect ratios, i.e., depth-to-width ratio, of two uniform-cross-section microchannels are 0.20 and 4.43 each with a hydraulic diameter of 33.7 μm and 36.4 μm, respectively. Another one is a diverging microchannel with a diverging angle of 0.183° and a mean hydraulic diameter of 34.8μm. All of the microchannels are made of SOI wafer and prepared using bulk micro-machining and anodic bonding. The surface roughness for both the bottom and the side walls was measured using an atomic force microscope enabling the explanation of convective boiling mechanism in the microchannel. The evolution of the eruptive boiling of water in the smooth microchannel was clearly examined using an ultra high speed video camera (up to 50,000 frames per second). It is confirmed that eruptive boiling is a form of rapid bubble nucleation after which the bubble merges with a slug bubble downstream in a short distance or evolve to a slug bubble. The bubble frequency in all of the cases studied is provided. Eruptive boiling may be predicted classically with micro-sized cavities that are consistent with the measured surface roughness. Furthermore, experiments are conducted to study the effect of channel cross-section design on boiling heat transfer in the microchannel. It is found that the slug bubbles tend to grow exponentially in the present microchannels. The results reveal that diverging microchannel presents better performance in boiling heat transfer than that of uniform-cross-section one, primarily due to more stable two-phase flow in the diverging microchannel. Empirical correlations based on convective boiling are developed, respectively, for both types of microchannel. For the same mass flow rate, the diverging microchannel presents higher single phase flow pressure drop, while the two-phase flow in both types of channels shows approximately the same pressure drop for boiling at the same heat flux. The aspect ratio of channels with approximately the same hydraulic diameter also affects heat transfer and pressure drop data significantly, single- or two-phase. For the same flow rate, the shallow microchannel presents better heat transfer performance in single phase region, while the deep one become better when boiling occurs. Analogically, the shallow microchannel depicts s higher single phase pressure drop and this may be due to EDL effect. While the two-phase flow in the deep one shows the smaller pressure drop for boiling at the same heat flux. Observation of two-phase flow pattern indicates that the flow reversal in the shallow microchannel is more violent than that in the deep one. This is consistent with the higher boiling heat transfer rate for the deep microchannel with a smaller two-phase pressure drop. The result of present study suggest that diverging cross-section design with a large aspect ratio is the better geometry for microchannels in a heat sink to have stable and high boiling heat transfer capability and yet low pressure drop.